Multimedia applications such as 2D/3D graphics, image processing, video compression/decompression, voice recognition algorithms and audio manipulation, often require the same operation to be performed on a large number of data items (referred to as "data parallelism"). Each type of multimedia application typically implements one or more algorithms requiring a number of floating point or integer operations, such as ADD or MULTIPLY (hereafter MUL). By providing macro instructions whose execution causes a processor to perform the same operation on multiple data items in parallel, Single Instruction Multiple Data (SIMD) technology, such as that employed by the Pentium.RTM. processor architecture and the MMx.TM. instruction set, has enabled a significant improvement in multimedia application performance (Pentium.RTM. and MMx.TM. are registered trademarks or trademarks of Intel Corporation of Santa Clara, Calif.).
SIMD technology is especially suited to systems that provide packed data formats. A packed data format is one in which the bits in a register are logically divided into a number of fixed-sized data elements, each of which represents a separate value. For example, a 64-bit register may be broken into four 16-bit elements, each of which represents a separate 16-bit value. Packed data instructions may then separately manipulate each element in these packed data types in parallel.
Referring to FIG. 1, an exemplary packed data instruction is illustrated. In this example, a packed ADD instruction (e.g., a SIMD ADD) adds corresponding data elements of a first packed data operand, X, and a second packed data operand, Y, to produce a packed data result, Z, i.e., X.sub.0 +Y.sub.0 =Z.sub.0, X.sub.1 +Y.sub.1 =Z.sub.1, X.sub.2 +Y.sub.2 =Z.sub.2, and X.sub.3 +Y.sub.3 =Z.sub.3. Packing many data elements within one register or memory location and employing parallel hardware execution allows SIMD architectures to perform multiple operations at a time, resulting in significant performance improvement. For instance, in this example, four individual results may be obtained in the time previously required to obtain a single result.
While the advantages achieved by SIMD architectures are evident, there remain situations in which it is desirable to return individual results for only a subset of the packed data elements.